1. Field of the Invention
This invention relates to corrosion resistant structural systems, and more particularly, to bimetallic systems in which steel is protected from corrosion resulting from intermittent contact with salt water by a protective non-ferrous overlay.
2. Brief Description of the Prior Art
Offshore drilling and oil production platform steel structural members are subject to severe corrosion at the location where these members pass beneath the surface of the water. This zone, referred to as the splash zone, presents a special problem in corrosion of the steel structural elements because they are here subjected to intermittent contact with salt water, salt-laden spray or mist, and air. The exposed surfaces are alternately wet and dry, and the sea water, as well as the mist, is saturated with oxygen. These conditions are conducive to rapid and severe corrosion of the steel structures.
Although effective protective coatings of organic composition can be applied to the steel members at locations elevated above the splash zone, and such members can be cathodically protected in the zone where they are always submerged beneath the water, neither type of protection is adequate within the splash zone. Considering the amount by which the level of the surface of the water may change with changing tides, and the height to which wave crests may extend above the mean surface of the water, the splash zone may, in some instances, range over 40 feet or more along the total vertical extent of the structural member. The problem of protecting the steel structural members located in the splash zone is intensified where joinder of such members by butt welds and the like is required.
Although it has been proposed for a number of years to protect steel structures exposed to corrosion in tidal and splash zones by providing a protecting sheathing of a corrosion resistant metal, as recognized in Morton et al. U.S. Pat. No. 2,791,096, it is there further explained that such protective sheathings themselves are difficult to apply to the steel structural member without engendering offsetting, and possibly more severe, problems than the corrosion itself. Thus, as the patentees in the Morton et al. patent point out, difficulty is experienced in attaching suitable corrosion resistant metal sheathing directly to the steel substrate by welding procedures, since special welds are necessary which even under the best conditions tend to weaken the steel substrate by migration of weld metal into the intermolecular interstices of the steel. Moreover, the sheathing material itself exhibits, in most cases, a propensity to crack during installation as a result of metal contamination. Morton et al. propose to overcome the described difficulties of sheathing the steel structures with a corrosion resistant protective metal by welding steel bands to the protective sheathing and also to the steel structural member substrate. Through the use of these intermediate bands, they allege that it is possible to avoid direct welding or attachment of the corrosion resistant metal sheathing to the steel structural members to be protected. A suggested sheathing material is a nickel-copper alloy. This arrangement is stated to obviate the danger of cracking of the sheathing materials as a result of drawing certain types of metals into the sheathing material to dilute and weaken the structural characteristics of the sheathing material.
It will be apparent from the description of the Morton et al. process as disclosed in the cited patent that avoidance of weld joints between the sheathing material and steel at locations still exposed to contact with sea water and air is not obviated, since the welds between the steel bands and the cladding or sheathing materials are still fully exposed to the corrosive environment.
The protection of steel or wooden pilings in a zone bridging the water line is considered in Fox U.S. Pat. No. 4,019,301 where the patentee advocates an enclosing inert sleeve of fiberglass, epoxy or similar material, with a space being left between the sleeve and the piling. This space is filled with an epoxy grout or the like which is allowed to set up and complete the protective sheathing or covering. No welding or securement of the sleeve and grout to the metalic or wooden substrate is advocated beyond the bond which may be established between the grout and the surface of the protected structural member.
In Shaw U.S. Pat. No. 3,719,049, it is proposed to protect the metallic structural member at the location where it traverses the splash zone by wrapping the structural member with a wrapper or jacket made of a flexible material, such as rubber or neoprene, and filling an annulus provided between this jacket and the structural member with a rust inhibitor. Constrictive steel bands are used at opposite ends of the jacket or wrapper to clinch the jacket in position on the structural member, and prevent its axial displacement therealong with resulting misalignment in relation to the splash zone. A generally similar proposal for protection of the structural member at the splash zone is also described in Lidell U.S. Pat. No. 3,996,757.
In a paper entitled "The Bimetal Concept", Welding and Metal Fortification, November 1974, pages 379-384, G. Newcombe points out that a number of practical and metallurgical problems are encountered in the fabrication of bimetallic structures in which a non-ferrous alloy is used for cladding or sheathing a ferrous substrate, such as steel, for purposes of corrosion protection. It is important that the structure be fabricated so that continued and extended integrity of the clad or sheath layer is realized. The author points out that steel and copper alloys have a significant potential difference in sea water, and that any defect in the joinder which provides a sea water leak path to the interface between the copper alloy and the steel will initiate severe and localized attack upon the steel. It is further important that the clad or sheathed layer not be terminated in a location exposed to sea water, since the steel immediately adjacent the clad layer will sustain severe selective attack.
It is also pointed out that where steel structural members are to be joined, such as by butt welding, copper alloy clad or sheath provided for protection at the surface of the steel members must be thoroughly removed from the zone adjacent the weld of the steel structures in order to prevent dilution of the steel by copper from resulting during the welding procedure, thus developing an extremely brittle martensitic zone at this point which subsequently results in cracking or structural failure. It is also important, in the welding procedure used in making the butt weld, that iron dilution of the outer protective alloy layer does not occur so as to impair its corrosion resistant characteristic. Such dilution also deleteriously affects the hot ductility behavior of the bimetallic structure produced.
In prior attempts to protect a splash zone-exposed steel substrate by the use of protectively applied nickel-copper alloys, a cladding or sheathing material from 0.050 to 0.25 inch thick has generally been used, and has been attached to the steel substrate by welding. Such bimetallic structures have not performed entirely satisfactorily. The welding involved in such systems is a difficult procedure, and it is very important to avoid mixing the steel and alloy material, since the problems described by Newcombe are then encountered. Moreover, the thin sheath provided in such methods is easily damaged both during fabrication, and later during service from floating objects that may be impacted upon the structural or piping members.
Another problem encountered in bimetallic structures of the type which include a non-ferrous metal sheath is that where the structure is a tubular hot product line, usually known as a riser, the different coefficients of thermal expansion which characterize the clad or sheath material from that which characterizes the steel tubing allows severe mechanical stresses to be developed in the structure as temperature changes occur. The coefficient of thermal expansion for steel is about 6.5.times.10.sup.-6 inch/inch/.degree.F., for nickel-copper alloys approximately 14.0.times.10.sup.-6 inch/inch/.degree.F., and for copper-nickel alloys, approximately 10.0.times.10.sup.-6 inch/inch/.degree.F. Considering the large vertical dimension which may characterize the splash zone as hereinbefore mentioned, it will be perceived that the differential expansion as between the sheath or cladding material and the steel tubular substrate can be significant in the case of such hot product line risers. This can result in severe buckling and rupture of the sheath, and this is aggravated unless the sheath has been carefully and properly bonded to the steel.